Finished
seamless steel pipes and precision steel pipes all have the problem of eccentricity and uneven thickness, but how does this occur? What causes eccentricity in seamless steel pipes such as cold-drawn steel pipes, cold-rolled steel pipes, and hot-rolled steel pipes? Why is the concentricity of seamless steel pipes not always ideal?
Eccentricity is most likely to occur during the hot-rolling process, primarily during hot piercing. Based on the dissection and analysis of steel pipes rolled by automatic tube rolling mills, we believe that after the pierced tube is rolled by an automatic tube rolling mill, the longitudinal and transverse wall thickness unevenness of the steel pipe basically retains the distribution characteristics of the uneven wall thickness of the pierced tube. That is, the rolled steel pipe still has a spiral-shaped wall thickness unevenness, and the transverse wall thickness unevenness is significantly increased.
First, the reasons for the uneven wall thickness of seamless steel pipes produced by automatic tube rolling mills are:
① The existence form and severity of the uneven wall thickness of the pierced tube directly affect the existence form and severity of the uneven wall thickness of the rolled steel pipe.
② During tube rolling on an automatic tube rolling mill, the bending of the mandrel causes the mandrel position to deviate from the center of the roll pass, resulting in uneven wall thickness. The wall thickness and minimum wall thickness positions on each cross-section of the tube at the beginning and middle remain almost constant; however, the unevenness gradually increases from the end to the beginning of the tube. Therefore, reducing the residual bending of the mandrel and decreasing the axial force on the mandrel during rolling significantly reduces wall thickness unevenness.
③ The greater the wall reduction, the more severe the wall thickness unevenness of the rough tube. A smaller wall reduction helps the automatic tube rolling mill reduce wall thickness unevenness in the pierced tube.
④ Incorrect roll pass adjustment, such as when the roll gap is not parallel, will exacerbate the wall thickness unevenness of the rough tube.
Second, ways to improve the wall thickness unevenness of seamless steel pipes:
① The spiral wall thickness unevenness distribution on the rough tube after secondary piercing (extension) is retained until the finished pipe. Therefore, improving secondary piercing (extension) is a key step in improving the wall thickness accuracy of the finished seamless steel pipe. The main measures are to improve tool design and increase the concentricity of the mandrel and the mandrel with the rolling line during rotation.
② Improving the wall thickness unevenness of the rough tube after piercing is an important step. The main measures are to improve the heating uniformity of the billet, improve the accuracy of the centering hole, lengthen the length of the mandrel leveling zone and the reverse cone, and improve the concentricity of the mandrel and the mandrel with the rolling line during rotation.
③ Although rolling produces severe symmetrical wall thickness unevenness, it has a certain effect on reducing spiral wall thickness unevenness. Therefore, rolling should be done in two passes, with the rough tube rotated 90° between passes.
④ The leveling process can basically eliminate symmetrical wall thickness unevenness, but its effect on eliminating spiral wall thickness unevenness is very small. Therefore, the capacity of the leveling machine should be improved.
⑤ The Fourier transform is an effective method for studying wall thickness unevenness during skew rolling. This method can also be used to study wall thickness unevenness in other steel pipe production units.
Third, precision-rolled steel pipes and cold-drawn steel pipes also experience eccentricity problems due to piercing:
Any factor that disrupts the correct geometry of the deformation zone formed by the rolls, mandrel, and guide plate will exacerbate the wall thickness unevenness of the tube.
(1) Mandrel.
① The shape design of the mandrel: Ideally, the rolling cone of the mandrel should be parallel to the exit cone of the roll. If the mandrel is designed according to the traditional Matveyev formula, its rolling cone is not parallel to the exit cone of the roll. The metal deforms within this gradually widening gap, inevitably causing insufficient rolling of the tube wall, resulting in uneven tube wall thickness. Moreover, the unevenness of the tube wall thickness worsens with the increase of the feed angle.
② Due to insufficient rigidity of the mandrel, bending occurs during piercing, preventing the mandrel from maintaining its center position, thus resulting in uneven wall thickness of the pierced tube.
③ Uneven wear or damage to the mandrel.
(2) Guide plates.
① If the guide plate distance is too large, the piercing centerline is maintained by the constraint of the guide plates during piercing. A large guide plate distance causes significant changes in the vertical position of the mandrel, making it unstable and leading to uneven tube wall thickness.
② Uneven wear of the upper and lower guide plates will also exacerbate the unevenness of the wall thickness.
(3) Rolls.
① Roll centerline misalignment: During production, incorrect installation of the screws on both sides of the piercing mill, or wear of the threads and bearings, can cause axial horizontal misalignment between the two rolls. Inconsistent feed angles between the two rolls lead to distortion in the deformation zone, resulting in uneven wall thickness.
② Large feed angles cause the mandrel and the rolling cone of the rolls to become less parallel.
③ Improper roll speed can also affect wall thickness accuracy.
(4) Billet centering and heating.
Eccentricity of the centering hole and uneven heating (yin-yang sides) will both cause uneven wall thickness.
(5) Rigidity, structure, and adjustment of the piercing mill.
Insufficient rigidity of the piercing mill body and unreliable locking mechanisms; inaccurate adjustment of the centering device of the push rod, unreliable operation, and excessive distance from the mill body; the adjustment of the rolling centerline, generally lower than the mill centerline to improve workpiece stability, can lead to asymmetrical changes in the relative relationship between tools in the deformation zone due to the shift of the rolling line, thus affecting the uneven wall thickness of the tube.
Therefore, eccentric thickness unevenness in seamless steel pipes cannot be completely avoided; it can only be strictly controlled step by step to minimize it.
